Method for cleaning gravel packs

ABSTRACT

A method for cleaning a plugged gravel pack in a subterranean wellbore is provided. The method comprises the steps of using a pressure pulsating jet and a tangential vortex to deliver a pressure pulsating treatment fluid into the gravel pack wherein soluble plugging materials in the gravel pack are dissolved by the treatment fluid and insoluble plugging materials are moved through the gravel pack and circulated out of the wellbore. The treatment fluid is driven more completely into the gravel pack to obtain a greater coverage in the pack to dissolve soluble fines and displace insoluble fines from the interstitial pore spaces of the gravel pack.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.10/256,736 entitled “Method for Cleaning Gravel Packs” filed on Sep. 27,2002, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method of cleaning plugged gravelpacks, gravel pack screens and perforation tunnels in a wellbore. Moreparticularly, it relates to a method for cleaning and/or removingplugging materials from a gravel pack completion without damaging thegravel pack material.

BACKGROUND OF THE INVENTION

Over time, most gravel packs will slowly lose permeability due to thereduction in pore space of the pack. This reduction in pore space can becaused in two ways. First, a scale can precipitate out of the well'sproduced fluids. In addition, fines can migrate out of the formation andbe trapped in the gravel pack. The pore spaces of the gravel pack becomeplugged with these precipitates or formation fines. These factors leadto an overall reduction in permeability, resulting in lower productionrates.

The plugging medium can potentially be removed from the gravel pack, bydissolving the plugging materials with chemicals or treatment fluids.However the insoluble plugging materials must be removed mechanically.

The present invention applies both chemical and mechanical techniques toclean a dirty, plugged gravel pack. It should be used whenever a gravelpack, screen and/or perforation tunnels exhibit signs of losingpermeability due to plugging. The present invention can be used toremove soluble and insoluble fines, precipitates, scales and asphaltenesthat can severely restrict the permeability of a gravel pack. Thus, thepresent invention satisfies a long felt need for a process capable ofcleaning plugged gravel packs by removal of soluble and insoluble fineswithout damaging the gravel pack.

SUMMARY OF THE INVENTION

According to the preferred embodiment of the invention, treatment fluidsare accurately placed through a gravel pack screen to treat a specificregion of a gravel pack, its perforation tunnels, the pack/formationinterface and the formation. Two preferred treatments include unpluggingthe pack by removing and/or dissolving fines and precipitates andplacing water control chemicals.

The treatment fluid is uniformly placed behind a screen into a sand orgravel pack by generating a tangential vortex and a localized yetfluctuating pressure gradient in the pack. A tangential vortex is acirculating current spinning about an axis substantially tangential tothe wellbore. The tangential vortex directs at least a portion of thereturn flow of the treatment fluid through the screen and up the gravelpack annulus before entering back through the screen. The efficiency ofplacing the treatment fluid is increased because the treatment fluid isreturned to the surface by way of the gravel pack annulus. Thefluctuating pressure gradient drives radial fluid flow through the pack.The fluctuating pressure gradient is achieved by the controlled rotationof a jetting nozzle operating at a flow rate sufficient to generate animpact pressure at the screen proppant interface, yet below apre-determined critical damage threshold pressure. As use herein, impactpressure shall mean the stagnation pressure of the jet on the surface itimpacts. The critical damage threshold pressure shall be understood tomean the pressure at which the impact pressure and the length of timethe pressure is applied, is great enough to break more than a smallpercentage of the proppant particles in the gravel pack. For example,API Recommended Practice 58, permits a maximum of 2% fines for gravelpack proppants in a sieve analysis.

The fluctuating pressure gradient causes the proppant to oscillate andthereby creating relative motion between the particles. This relativemotion, not only increases the rate at which treatment fluid can invadea pack but increases the rate at which particles can be mobilized intothe flow stream to be transported out of the pack. The oscillating ofthe proppant particles reduces the friction between the fines andproppant. The forces created by the viscous drag of the fluid on thefine particle can more easily remove it from the pack.

In addition, the energy level of the oscillating pressure at higherimpact pressures is enough to abrade deposits off the surface of theproppant particle. The higher strength of man-made proppants canaccommodate these high energy levels. Therefore, there are twomechanisms at play to remove fines and precipitates from the pack, firstto dissolve the soluble fine particle by chemical means, the other is toabrade the precipitate off the proppant, reduce friction between theparticles thereby increasing the rate the particles will mobilize intothe flow stream, along with other non-soluble fines and transport themout of the pack.

Rotational motion of the nozzle creates pressure pulsing, which has alsoproven to be an effective way to clean unwanted deposits (e.g., scales,waxes and asphaltenes) off the inside diameter of the screen face.

A method of uniformly placing a treatment fluid behind a screen in agravel pack in a wellbore according to one embodiment comprises thesteps of delivering a pressure pulsating jet of treatment fluid througha jet nozzle against the screen, and creating a tangential vortexbeneath the jet nozzle with the treatment fluid wherein at least aportion of the treatment fluid is directed through the screen and intothe gravel pack before returning to the surface. The jet nozzle may beoriented to have an axial downward component to the jet direction. Anannular region of slurry with low proppant concentration is createdbehind the screen wherein the flow rate of the treatment fluid in theupwards direction in the annular region is maintained above thethreshold transport velocity to suspend the proppants in the annularregion.

A method of cleaning a gravel pack in a wellbore according to anotherembodiment comprises the steps of positioning a pressure pulsating jetinside a gravel pack screen, delivering a pressure pulsating treatmentfluid into the gravel pack through the gravel pack screen with apressure pulsating jet, dissolving soluble plugging materials in thegravel pack with the treatment fluid, and moving insoluble pluggingmaterials through the gravel pack and circulating the insoluble pluggingmaterials out of the wellbore. The method further comprises displacingthe treatment fluid with a displacement fluid with the pressurepulsating jet. In another embodiment of the invention, a method ofwashing a plugged or partially plugged gravel pack and wellborecomprises the steps of delivering a pressure pulsating treatment fluidinto the gravel pack, dissolving soluble fines located in theinterstitial pore spaces of the gravel pack with the treatment fluid andreducing the pressure drop as a fluid flows into and through the pluggedor partially plugged gravel pack by oscillating the fines contained inthe gravel pack with the pulsating fluid. The method further comprisesoscillating insoluble and yet undissolved fines located in theinterstitial pore spaces of the gravel pack with the pressure pulsatingtreatment fluid until the insoluble fines move through the gravel pack,and circulating the insoluble fines out of the wellbore. Full coverageof the gravel pack is provided by controlling the flow rate, rate ofpenetration and impact pressure. Damage is prevented by controlling theimpact pressure and flow rate to correspond to the specific gravel packdesign. The reaction time between the treatment fluids and the fines andgravel particles may be controlled to prevent damage to the gravelparticles. Thus, one is able to pump treatment fluids that will reactwith both the plugging fines and gravel particles, but because thesurface area to volume ratio of the fines is much higher, the fines canbe substantially dissolved without damaging the gravel particles.

According to another embodiment of the invention, a method of cleaning agravel pack in a wellbore is provided comprising the steps ofpositioning a pressure pulsating jet inside the gravel pack, deliveringa pressure pulsating treatment fluid into the gravel pack with thepressure pulsating jet and creating a radial pressure gradient withinthe gravel pack as the pressure pulsating jet is moved through thegravel pack.

One aspect of the invention is directed to a method of washing a gravelpack in a wellbore comprising the steps of delivering a pressurepulsating treatment fluid into the gravel pack with a pressure pulsatingjet and a tangential vortex and dissolving soluble plugging materials inthe gravel pack with the treatment fluid.

Another aspect of the invention is directed to a method of washing agravel pack in a wellbore comprising the steps of delivering a pressurepulsating fluid into the gravel pack with a pressure pulsating jet and atangential vortex, moving insoluble plugging materials through thegravel pack with the fluid and circulating the insoluble pluggingmaterials out of the wellbore.

The pressure pulsing of the present invention is an improvement overprior jetting systems. The pressure pulsing vibrates plugging materialsin the gravel pack. This oscillating movement and/or vibration leads togreater efficiency in delivering treatment fluids deeper and morecompletely through a gravel pack and into the perforation tunnels. Theappropriate impact pressures utilized by the present invention providesufficient energy to oscillate the fines yet not damage the gravel pack.Thus, production may be improved by dissolving soluble fines andremoving insoluble fines from the pore spaces of the pack. Additionalobjects, features and advantages will be apparent in the writtendescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these figures in combination with the detailed description ofthe specific embodiments presented herein.

FIG. 1 illustrates a pressure pulsating jet washing a gravel pack in awellbore.

FIG. 2 is a section view of the pressure pulsating jet of FIG. 1.

FIG. 3 illustrates the tangential vortex created by a pressure pulsatingjet to wash a gravel pack.

FIG. 4 illustrates a fine plugging the interstitial pore space betweensand particles in a gravel pack.

FIG. 5 illustrates the fine of FIG. 3 after it has been oriented forpassage through the interstitial pore space of the gravel pack by thepulsating treatment fluid of the present invention.

FIG. 6 is a graph of cumulative pack removed versus the number of passesfor a pressure pulsating jet at various impact pressures on man madeproppants and sand gravel packs.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventors to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

In one embodiment of the present invention, a solvent, acid treatment orenzyme treatment, is provided to remove the soluble materials from adirty, plugged gravel pack. The key to the treatment is the chemicalsare not simply bullheaded into the well. The chemicals are placed andremoved from the gravel pack in a controlled and optimized mannerpreferably using coiled tubing and a pressure pulsating jettingapparatus such as the Roto-Jet™ tool offered by BJ Services Company.

The Roto-Jet™ is a pulsating pressure jetting apparatus which when usedin accordance with the present invention forces the treatment fluidsinto the pore space of the gravel pack. The chemicals are driven into avery localized area of the pack and not just indiscriminately pumpedinto the well bore, thereby finding the path of least resistance. Thisis achieved by a high velocity pulsed jet directed precisely into thegravel pack. The kinetic energy of the pulsing fluid delivers thetreatment fluids through perforated base pipe, through the wire wrappedscreen and through the gravel pack and into the perforation tunnels. Inthe same manner in which the treatment fluid is placed it can bedisplaced (thereby removed) by another treatment fluid or anon-treatment fluid to flush out the original treatment fluid. Again theaccuracy with which the fluid can be placed and the ability to forcefluid into the perforation tunnels is the key. The flow rates, rate ofpenetration (“ROP”), pulse rate, and jet pressures are controlled in amanner to ensure complete coverage of the gravel pack by the treatmentfluids. Not only is this process more effective as it ensures maximuminteraction between the soluble fines and the treatment fluid, but isalso more economical. A controlled volume of treatment fluid may beprecisely placed as opposed to an indiscriminate volume flowing to thepath of least resistance.

The second advantage of this pressure pulsating system is the removal ofinsoluble fines. In some instances, a large percentage of the materialsplugging the gravel pack can be insoluble and in this situation achemical reaction can not be the primary treatment process. A pulsatingjet, such as created by the Roto-Jet™, hydraulically oscillates theplugging fines within a gravel pack, ultimately transporting theinsoluble fines (as well as any yet to be dissolved soluble fines) outof the pack where they may be circulated to the surface and out of thewell. The pulsating jet mobilizes the fines, dislodging them fromin-between the sand particles of the gravel pack. A momentum exchangebetween the pulsating fluid and the solid matter in the gravel packoccurs. This momentum exchange causes both the pack sand particles andthe plugging solids contained therein to oscillate. This vibrationmobilizes the plugging fines into the circulating currents set up by thetangential vortex. However, the hydraulic power (i.e., the flow ratemultiplied by the impact pressure) of the pulsating jets must becontrolled to ensure there is minimal damage to the pack. If thehydraulic power is too high it is possible to break up and remove packsand, thereby creating voids. Voids that are not subsequently filled bythe excess sand (or proppant) originally deposited above theperforations can lead to produced well fluids jetting and eroding a holein the wire wrapped screen, resulting in a failed gravel pack. If thehydraulic power is too low the process suffers loss in efficiency.

Excessive fluid “jet-velocity” can cause excessive particle oscillationleading to sand grain fragmentation and abrasion leading to thegeneration of “fines.” “Fines” will be flushed out of the pack andthrough the screen. However, grain fragmentation is not an isolatedevent and repeated “cycles” will continue to generate “fines” and overtime the pack volume will diminish. Rate of generation of “fines” (i.e.,pack contraction) can be minimized to negligible levels by controllingthe “impact pressure” and the number of times the nozzles passes by awellbore location. The compromise is to reduce impact pressure andnumber of passes such that negligible levels of fines are generated.Preferably, the level of fines left behind will be no more than theminimum level as allowed by API Recommended Practice 58, “RecommendedPractices for Testing Sand Used in Gravel Packing Operation.”

It is the above described combination of chemical and mechanical methodsthat can effectively remove both types of blockages from a gravel pack,and with proper implementation, there is little risk of damaging thegravel pack. It will be appreciated that the described methods are alsoeffective in removing such blockages from a frac pack. The presentinvention will deliver treatment fluids to the fractures extending fromthe perforation tunnels. For the purposes of brevity, the term “gravelpack” as used herein will include both gravel packs and frac packs, asthose terms are used in the art.

According to a preferred embodiment, controlled “pulsing” throughrotation causes oscillation of the proppant and plugging materials.Pulsing and a tangential vortex allows fluid to invade the plugged packmore quickly than other processes. Treatment fluid can thus beintroduced into the pack and reach the site of the plugging material anddissolve it more efficiently.

Fluid invasion into the pack is highly localized in the axial directionand extends radially in the vicinity of the tool nozzles and in theregion immediately below the nozzles by the tangential vortex (FIG. 3).Therefore the process can be controlled such that the entire pack,perforation tunnels, and/or pack-formation interface receives treatmentfluids.

Fluid can be accurately placed and subsequently removed/flushed away.Therefore, not only can the process deliver full coverage but thetreatment time can be controlled as well. With formations, proppants ortubulars that are sensitive to treatment fluids, the Roto-Jet™ can washthe treatment fluid out of the gravel pack or near wellbore, yieldingless potential for corrosion or secondary precipitate damage. Therefore,reduce treatment costs can be achieved by reducing treatment volumes andreducing/optimizing treatment time (thereby reducing rig time).

To better understand the present invention, one needs to consider how agravel pack gets plugged with insoluble non precipitated fines. If apack consisted of perfectly spherical particles of exactly the samesize, it would be very difficult for particulate material to stop midwaythrough the pack and get stuck in the pore spaces and therefore plug thegravel pack. The reality however is the gravel pack sand particles arenot perfectly spherical and range in size. This leads to tapering flowchannels and therefore a greater propensity to plug. For example, acommon gravel pack sand is a 20/40 mesh with pore space sizes that willpermit spherical shaped particles ranging in size from 0.0025″ diameterto 0.0051″ in diameter to pass through. Any particles larger than thiswill not enter the pack and particles smaller than this have a greaterprobability of passing through the gravel pack unimpeded. Therefore,particles of this size plugging the pack can pass through the pack ifthere is only small differential movement between the gravel pack sandparticles when the pack is agitated by a pressure pulsating jet. Inaddition, irregular shaped particles can also be mobilized out of thegravel pack by oscillating the particles. Oscillation will reducefriction between the fines and the proppant particles, the fines canthen rotate about their own axis into an orientation, that will allowthe fluid flow to transport the fine particles through the pore spacesin the gravel pack.

FIG. 1 illustrates one embodiment of the present invention. A pressurepulsating jet apparatus, such as the BJ Services' Roto-Jet™, is showncleaning a gravel pack. The pressure pulsating jet apparatus 10 ispreferably run into the gravel pack attached to a coiled tubing (notshown). However, it will be appreciated by those skilled in the art thatthe pressure pulsating jet apparatus could be run on a conventionalworkstring. Preferably, the pressure pulsating jet apparatus, such asthe Roto-Jet™ is run inside of the existing production tubing 15 pastliner hanger 20 and into gravel pack screen 25, which is suspended fromthe production casing. A standard Roto-Jet™ has an outer diameter of 1¾inches. Thus, the preferred rotary jetting tool will easily pass throughtypical production tubing, packers and screens. Screen 25 may be aconventional wire-wrapped base pipe screen or a commercially availablepremium screen such as Baker Oil Tools' Excluder™ screen or WeatherfordCompletion Services' Stratapac™ screen. Such screens are well known inthe art. Alternatively, a slotted liner may be used with a gravel pack,or a pre-packed screen may be used, as is well known in the art. Forpurposes of this invention, it will be understood that such devices willcollectively be referred to as a gravel pack screen. Sand particles 70are packed in the annulus between screen 25 and casing 35. Common gravelpack sands which may be cleaned by the present invention include 12/20mesh, 20/40 mesh and 40/60 mesh size particles. Such naturally occurringsands are available from Accumen or Badger mines. Gravel packs usingsimilar sized man-made gravel, such as the commercially availableCarboLite™ particles, manufactured by Carbo Ceramics, or Econo-prop,also manufactured by Carbo Ceramics, may also be cleaned by the presentinvention.

A plurality of perforations 40 provide communications between thesurrounding hydrocarbon bearing formation and casing 35. Perforationtunnels 45 extend from the casing and through the surrounding cementsheath (not shown) and into the adjacent formation 50. The perforationtunnels are also packed with sand particles 70. As the pressurepulsating jet is lowered through the gravel pack, the jet 55 delivers apressure pulsating fluid through the gravel pack screen, into the sandparticles 70 of the gravel pack, and into the individual perforationtunnels.

Preferably, the bottom hole assembly providing the pressure pulsatingjet is acid compatible. The preferred pressure pulsating jettingapparatus, the Roto-Jet™, uses a multi-stage fluid turbine (not shown)as an internal drive mechanism to drive mole 30 which spins a pluralityof jet nozzles 32 mounted on the mole. The Roto-Jet™ includes a speedgovernor to control the speed at which mole 30 rotates. The fluidturbine is actuated by pumping fluid through the various stages.Typically, the fluid would pass through a downhole filter section at thetop of the tool and then enter the turbine. The entire volume flow ratemay be directed through the turbine blades or a portion of the flow maybe directed through the center of the turbine shaft to adjust therotational speed of the mole, as discussed below. The combined flow thenpasses into the jetting mole fixed to the bottom of the turbine shaft,and leaves the tool via the plurality of jet nozzles mounted in themole. Radial and thrust bearings are located near each end of theturbine shaft to handle the thrust forces acting on the tool. A rotaryspeed governor located immediately below the downhole filter is coupledto the turbine shaft. The governor controls the speed of the turbine byapplying a drag torque which varies in proportion to speed. The dragtorque is applied by a series of magnets radially spaced about the shaftof the governor. Rotational speed of the mole, and thus the jet nozzles,may be adjusted by altering the number of turbine stages, altering thenumber of magnets in the governor or changing the size of an orificewhich controls the amount of fluid that may be diverted through thecenter of the turbine shaft. The rotational speed of the mole can beslowed by removing turbine stages, adding additional magnets to thegovernor or by diverting more fluid through the turbine shaft using alarger shaft bypass orifice. Conversely, rotating of the mole can beincreased by increasing the number of turbine stages, removing magnetsfrom the governor or by diverting less fluid through the turbine shaftusing a smaller shaft bypass orifice. Thus, for any given gravel packwashing, the desired rotational speed of the pulsating jet nozzles maybe determined in advance and corresponding alterations to the jettingtool may be made at the surface before running the tool into thewellbore.

In a preferred embodiment, the Roto-Jet™ has a pair of jet nozzlesspaced 180° apart and oriented at a 75° angle from the axis of the toolfor man made proppants. When activated, the jet nozzles spin around atspeeds preferably ranging from about 100 rpm to about 800 rpm.Rotational speed of about 400 rpm is the optimum speed for scaleremoval. The repeated and rapid passage of the jet-stream from eachnozzle creates a pressure pulsating radial pressure gradient throughoutthe treated area. Once the jet stream from a nozzle passes a givenpoint, the fluid pressure dissipates until the next jet nozzle passesthe same point. In this way, the Roto-Jet™ delivers a pulsating, on/off,pressure radially inside the gravel pack.

The optimum tool set-up for gravel pack cleaning can be achieved with atangential vortex. As shown in FIG. 3, the tangential vortex is acirculating current spinning about an axis substantially tangential tothe wellbore. With two opposing jets, there is an axial component forthe axis of the vortex.

To establish a tangential vortex the jets 55 have an axial downwardcomponent to the jet stream direction, the downward component of the jetstream direction either directly or after the jet as struck the innerdiameter of the base pipe. The jets thereby cause fluid to flow down thewellbore for a distance before the flow decelerates, stops and returnsback up the wellbore (represented by arrow tails 61). Depending on thestrength, or energy level of the jets, some of the fluids near the jetare entrained in the jet stream, as shown by arrows 59. Therefore, thejets and the fluid entrained causes a tangential vortex, represented bythe arrows 55, 60, 61, 62 and 59.

Further, if the energy levels of the jets are large enough, such thatthe cross sectional area of the jet(s) stream is large enough as itdiverges, then a significant portion of cross sectional area of the basepipe will contain jet(s) with fluid flow in the downward direction.Therefore the cross sectional area 60 of the jets, at a given throwdistance, is large enough to restrict the returning fluids from flowingup the wellbore inside the base pipe. The path of least resistancebecomes the path through the screen and into the gravel pack annulus 53,as illustrated by arrows 62. If the flow rate in the annulus is greatenough and in the upward direction (i.e., greater than the thresholdtransport velocity of 4 inches/second) the flow will suspend andtransport the proppant particles. Since the flow is in the upwarddirection the proppant particles are suspended against the pull ofgravity.

As the bottom hole assembly (BHA) is lowered or lifted in the wellbore,an annular region 57 of slurry with low proppant concentration develops.The packing density of the proppant particles is less than the maximumpossible body centered cubic stacking, and therefore void spaces existin the gravel pack. Therefore as the BHA is lowered or lifted in thewellbore, the void spaces can be captured by the fluid with a velocityabove threshold transport velocity. As the BHA travels up or down thewellbore the accumulation of void space can grow up to a maximum volume,the region of annulus with flow rates above the threshold transportvelocity. This region has low proppant concentration slurry.

Under these conditions, the efficiency of placing treatment fluids intothe pack is dramatically increased. In the absence of a tangentialvortex the pumped treatment fluids can return to surface withoutentering the gravel pack annulus. The efficiency is therefore increasedbecause the fluid flowing from the jets flows into the annulus, insteadof flowing back up inside of the base pipe, thereby returning to surfaceby way of the annulus 53 of the gravel pack. In addition, the flow ratein the annulus is great enough to suspend proppant particles. This leadsto an annular ring of slurry with low proppant concentration, furtherreducing the restriction to the flow. In this way, the annulus isflooded with pumped fluids or treatment fluids. Further, some of thetreatment fluid is circulated more than once through the annulus, as itis re-entrained into the jet stream. The proppant particles are therebythoroughly washed with reactive chemicals. Since, most of the expensivetreatment fluids pass through the annulus, the cost of the treatment canbe reduced. Further the time to place treatment fluids into the pack canbe reduced, further improving the economics.

Another benefit of this process is a balance can be achieved for thehydraulic power of the jet stream between the impact pressure and thetangential vortex. Decreasing the jet angle, as illustrated in FIG. 3,decreases the impact pressure for a given flow rate and jet pressure,but increases the flow rate and velocity of the fluid in the downwarddirection inside the base pipe thereby increasing the strength of thetangential vortex. Decreasing the jet angle, decreases the rate ofdamage to the proppant particles. Therefore an optimum jet angle can bedetermined, such that damage to the proppant particles is minimized butenough hydraulic power is supplied to create a tangential vortex andpressure oscillations.

Therefore a tangential vortex can deliver a large fraction of the pumpedfluids into the gravel pack annulus. A tangential vortex can alsocirculate fine particles, whether produced fines, or small particles ofprecipitate broken up by the pulsating jets out of the gravel pack.Gravel particles will not circulate out of the pack as the screen trapsthem in the annulus. The low concentration proppant slurry regiontransports fines out of the pack, since this region is predominantlyfluid, proppant particles do not impede the fines from travelling out ofthe annulus. Proppant damage is also minimized with a tangential vortexas low hydraulic power is required to flow treatment fluid into the packand to transport fines out of the pack.

The rotating action of the Roto-Jet nozzles provide full wellborecoverage. A variety of jet nozzle configurations and number may be used.For example, the jet nozzles may be configured to have two opposing jetswith a radial 90° discharge angle with the longitudinal axis of thetool. This angle is optimum to achieve a maximum impact pressure for agiven nozzle pressure and flow rate, thereby generating the largestpressure pulsation in the gravel pack. However for a tangential vortex,the 90° discharge angle is the least preferred. When the jet impacts theinside of the base pipe, only half the flow has the potential to travelin the downward direction. Only flow in the downward direction sets up atangential vortex. Therefore in the preferred embodiment the dischargeangle for man made proppants is about 75°. This enables strong impactpressures and enough downward fluid rate and momentum to create thetangential vortex. Since man made proppants are stronger and moreconsistent in strength than naturally occurring proppants, higher impactpressures can be used with man-made proppants than naturally occurringproppants. Reducing the discharge angle for a given hydraulic powerreduces the impact pressure, yet increases the fluid flow rate andvelocity in the downward direction. Therefore reducing the dischargeangle can be used to lower the impact pressure and increase the strengthof the tangential vortex. For example, a 75° nozzle with a jet stream of100 liters/min. and nozzle pressure of 1200 psi does not have thehydraulic power to create the tangential vortex and has an impactpressure of 200 psi. An impact pressure of 200 psi damages the naturallyoccurring sand in a gravel pack. If the same flow and pressure isdirected through a nozzle at 45°, the impact pressure reduces to 70 psiand there is enough downward flow rate and velocity to power thetangential vortex and dramatically reduce the rate of damage. Atangential vortex may be created using a single jet nozzle or an arrayof several jet nozzles.

A variety of jet orifice is available to optimize the impact pressureand hydraulic horsepower to be applied to the pack. Common sizes include0.110 inches, 0.119 inches, 0.126 inches, 0.141 inches and 0.161 inchesin diameter.

A substantially plugged gravel pack can not receive sufficient treatmentfluid into the pore space of the gravel pack to dissolve soluble fineslocated in the pore spaces. Because the pore spaces are not 100%plugged, there is still some pore space to let fluid in but withoutmobilizing the fines and moving them within the pore space the abilityto deliver the treatment fluid into the entire pack is severelyhandicapped. Inducing a pulsating action in the fluid flow allows thetreatment fluid to drive completely into the pack to obtainsubstantially full coverage of the gravel pack thereby increasing theability to remove both the soluble and insoluble fines. The pressurepulsing allows the acid or solvent fluids to flow deeper into the gravelpack and into the perforation tunnels due to the decrease in flowresistance through the gravel pack. Pressure pulsing reduces frictionbetween proppant particles to aid in the creation of the tangentialvortex. The reduced friction between the proppant particles created bythe pressure pulsing also reduces the angle of repose, therebyincreasing the proppant packing density in the annulus and in theperforation tunnel.

Pressure pulsing can break the bonds between the proppant particles andany cementatious precipitate in the gravel pack. Pressure pulsing alsoremoves unwanted deposits such as scale, waxes and asphaltenes from theinner diameter of the screen. Thus, it is possible to remove unwanteddeposits (e.g., scale) from the screen and then remove pluggingmaterials from the gravel pack in a single trip. The rotating jet(rotating in the direction indicated by arrow 64 in FIG. 3) increasesthe quantity of wellbore fluids entrained into the jet and provides fullcoverage of the gravel pack.

The pressure pulsing also causes relative movement between the pluggingfines and the sand particles and this in turn permits insoluble ornon-dissolved solids to move through and ultimately out of the pack.This mechanical removal of plugging materials from the pack isillustrated in FIGS. 4 and 5. FIG. 4 shows a plugging fine 75 trapped inthe pore space between sand particles 70. Fine 75 is blocked from movingthrough the gravel pack because of its orientation. FIG. 5 illustratesfine 75 after its orientation has been changed due to movement caused bythe pressure pulsing treatment fluid. As shown in FIG. 5, fine 75 is noworiented so that it can pass through the pore space between particles70.

Once the pressure pulsing fluid has been delivered through the gravelpack screen and into the gravel pack and perforation tunnels, the fluidthen recirculates back through the screen 25 and perforated base pipe26. A portion of the returning fluid 59 will be entrained in the jetstream and be recirculated by the jet stream back through the gravelpack annulus. The remaining fluid 58 will begin flowing up the annulusbetween the tool and inner diameter of the perforated base pipe 26.Ultimately, the insoluble and non-dissolved particles are circulated outof the well with the main fluid flow, shown by arrow 100 in FIG. 3.Thus, the pulsating fluid is driven into the pore spaces and dissolvesthe soluble fines while also mobilizing the insoluble fines to allow thelatter to be flushed out of the gravel pack where it can be circulatedout of the wellbore by the displacing fluid.

A pressure pulsating jet can deliver a pulsating jet at a controlledpressure into the gravel pack without damaging the gravel pack. Thepulsating jet mobilize and displace the fines in the interstitial porespaces in the gravel pack. Once the fines are mobilized, the treatmentfluid can penetrate the gravel pack more efficiently.

In one embodiment of the invention, the treatment fluid is an acid suchas hydrochloric acid, hydrofluoric acid or organic acids, such as aceticacid and formic acid, or combinations of these acids. Other acidssuitable for use with the invention include acids such as Sandstone™acid, available from BJ Services Company, and self generating acidsystems. In another embodiment, the treatment fluid is a solvent.Suitable solvents include xylene, diesel alcohols, aromatic andnon-aromatic hydrocarbons, and surfactant systems, as well ascommercially available products such as Paravan™, available from BJServices Company. In another embodiment, the treatment fluid includesnon-acid reactive systems such as enzymes, bleach and oxidizing systems,chelating agents and combinations of these materials. One of skill inthe art will also understand that some pumping schedules could involvestages of solvents followed by acid or solvents followed by non-acidreactive systems. Stages could be either single or multiple depending onthe nature of the plugging problem.

The treatment fluid may also be a water control chemical. Placing thesechemicals with the present invention may yield more effective treatmentsand allow optimization of treatment volumes.

The treatment fluid, such as acid, may be displaced with another fluid,such as water, sea water or KCl water. The displacement fluid iscirculated by the pulsating jet and the tangential vortex into thegravel pack screen, the gravel pack itself, and the perforation tunnelsand circulates the liberated insoluble fines out of the wellbore. Thetime a treatment fluid is permitted to soak, or remain in the gravelpack can now be controlled. Highly treatment fluids can now beconsidered since the reaction time can be controlled. Although thepresent invention is particularly well-suited to wash and clean a gravelpack in a cased hole, the invention can also be used to wash and cleanopen hole gravel packs.

Perforation tunnels are packed with proppant as fluid transports theproppant particles during the placement of the pack. However,perforation tunnels that are plugged by a drilling skin or by othermechanisms will receive little proppant as the fluid can not flow intothe perforation tunnel and therefore can not transport the proppant. Thepulsing action first helps to place treatment fluids into theperforation tunnel to remove the damage and then efficiently packs thetunnel with proppant by the mechanism described above. Bridging can alsooccur during a gravel pack. The pulsating treatment fluid describedabove can remove such bridges.

It is important not to break down or erode the proppant of the gravelpack, which would lead to a lower permeability of the pack. Thus, thepreferred embodiment of the invention contemplates the delivery of thetreatment fluid using appropriate impact pressure to avoid breaking ordestroying a gravel pack particle. The maximum impact pressure thegravel pack can sustain is a function of proppant type, pack tightness,screen type, and proppant size. Through experimental testing the maximumimpact pressure can be determined as a function of these variables. Oncea maximum impact pressure is determined, flow rate, and rate ofpenetration can be optimized to ensure full coverage and optimum volumeof treatment fluid per meter of interval.

By way of example, FIG. 6 is a graph of the cumulative pack removed (asa percentage of the pack jetted on) versus the number of passes throughthe test pack by the pulsating jet. The cumulative pack removedrepresents the damage caused by the breaking down or erosion of theproppant of the gravel pack due to the impact pressure. The damagedproppant is removed from the pack as a fine. FIG. 6 compares variousimpact pressures at 400 rpm on a vertical, pumped in, clean gravel packwith either 30/50 mesh Enono-Prop or 40/60 mesh Ottawa sand, a naturallyoccurring proppant. A 1% damage threshold was selected. This representedone half of the minimum level of fines allowed by the API RecommendedPractice 58.

As can be seen in FIG. 6, for example, the cumulative damage caused byfive passes at an impact pressure of 273 psi in a sand gravel pack isapproximately ten times the damage caused to an Econo-Prop for the sameparameters. Thus, excessive damage to the pack would be created by animpact pressure of 273 psi for the naturally occurring sand particles.Conversely, such an impact pressure would not cause excessive damage toa comparable wellbore packed with Econo-Prop.

Preferably, the critical damage threshold to the pack would be about 1%or less. However, the tradeoff for accepting a higher damage thresholdis a more thorough cleaning of the gravel pack. Again, using FIG. 6 asan example, making 8 passes of the tool at an impact pressure of 826 psithrough an Econo-Prop pack would remove about 2.0% of the pack comparedto about 1.0% of the pack for 4 passes of the tool. Depending on theparticular well, the more thorough cleaning of the pack by theadditional passes of the tool may be worth the additional damage to thepack. Thus, a damage threshold of up to about 3%, for example, may bejustified by the resulting cleaner pack. However, if too much proppantis removed from the gravel pack, the upper perforations may not bepacked with gravel which could lead to the destruction of the gravelpack screen and the loss of the gravel pack filter.

During experimental testing it was determined that the maximum impactpressure that could be sustained by a pack was a function of the packpacking density. Therefore testing was done on packs that had a packingdensity or “tightness” representative of what is found in the typicaloil & gas wells. However, after a treatment was placed with theRoto-Jet™, pack tightness increased. Pack density increases typically5-10% by this process. This is due to the pressure oscillating of thefluid, reducing the angle of repose of the proppant particles, anddecreasing the friction between the proppant particles. Therefore,higher packing density can be realized.

Depending upon the particulars of a given gravel pack, the pressurepulsating treatment fluid may be delivered at impact pressures, forexample, ranging from about 5 psi to about 850 psi without damaging thepack. However, even with low to moderated impact pressures, the presentinvention provides for a more efficient placement of treatment fluidsinto a gravel pack. As a result, less time in the wellbore is needed toplace the treatment fluid. The treatment fluid can be driven into thegravel pack quicker. Consequently, less treatment fluid is required tobe pumped than with previous known methods of gravel pack washing. Sinceless time and less acid are required, the overall cost of washing aplugged gravel pack can be reduced with the present invention. By way ofexample, acid treatments on the order of about 40 liters/meter of gravelpack to about 400 liters/meter of gravel may be used with the invention.However, one of skill will appreciate that the volume of acid requiredto wash a gravel pack will depend on the size of the gravel pack.

In a preferred embodiment, an operating envelope has been establishedfor the most effective system for cleaning gravel packs without causingpack damage. According to preferred method, the pressure pulsating jetis lowered through the gravel pack at a speed of about 0.2 meters perminute to about 10 meters per minute. Again, one of skill willunderstand that the speed of running the tool through a given gravelpack will depend on the particulars associated with the pack, such asthe size of the gravel pack, the type and size of the gravel, and thedimensions of the downhole tubulars. Preferably, the treatment fluid isdelivered to the gravel pack at a flow rate of about 40 liters perminute to about 400 liters per minute.

The following examples further illustrate the treatment of typicalgravel pack configurations in accordance with embodiments of the presentinvention. It should be appreciated by those of skill in the art thatthe treatments and/or configurations disclosed in the examples whichfollow represent treatments and/or configurations discovered by theinventors to function well in the practice of the invention, and thuscan be considered to constitute preferred modes for its practice.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments which are disclosed and still obtain a like or similarresult without departing from the scope of the invention.

The following examples, as well as FIG. 6, are representative of testsconducted with a gravel pack cleaning test apparatus which was built tosimulate downhole conditions. The test fixture was designed and built toinclude an axial feed system to simulate the rate of penetration (bothlowering and raising) a bottom hole assembly which included the pressurepulsating jet. The fixture was adjustable to accept different sizes ofcompletions and allowed the bottom hole assembly to be setconcentrically or eccentrically with the simulated wellbore. A highpressure pump was attached to the fixture to allow fluid injection andpressures up to 6500 psi. In one embodiment, the fixture was fitted witha 6 inch outside diameter acrylic tube with a quarter inch thick wall tosimulate the casing. Simulated perforations {fraction (3/4)} inch ininside diameter and 9 inches long were placed on the casings at 6 inchintervals. A 3½ inch base pipe wire wound screen was mounted inside thecasing as per standard gravel pack. An alternative embodiment of thetest fixture included a 7 inch outside diameter acrylic casing having aquarter inch wall was used with a 4 inch diameter base pipe screen. The7 inch casing included the same perforation tubes as the 6 inch diametercasing. The annular volume between the screen and the casing was filledwith gravel pack proppant and compressed by an annular piston to tightenthe pack. The simulated gravel pack was approximately 56 inches inoverall length. The test fixture could be oriented at various anglesfrom vertical to horizontal.

EXAMPLE 1

A 2.125 inch outer diameter tool equipment with R90C nozzles (i.e., 90degree, 0.126″ diameter nozzle) is used to treat a typical Gulf ofMexico gravel pack (3.5 inch outer diameter perforated tubing as basepipe, with a 3.9 inch outer diameter wire wrapped (0.008 inch gap)screen, inside 7 inch casing (6.276 inch inner diameter) and 40/60sand). The standoff to the outer diameter of the base pipe (or innerdiameter of the wire wound screen) is 0.97 inch for the Roto-Jet™ whenthe bottom hole assembly is centralized. Typical flow rate for thisconfiguration of Roto-Jet is 110 liters/minute results with a pressuredrop across the nozzle of 1021 psi. The impact pressure at the innerdiameter of the wire wrapped screen is 361 psi.

EXAMPLE 2

A fully eccentric 1.75 inch tool with R90C nozzles is used to treat thetypical pack described in Example 1. All other parameters are the sameas Example 1 (3.5 inch outer diameter base pipe, 3.9 inch screen outerdiameter 6.276 inch inner diameter casing and 0.008 inch gap screen with40/60 sand at 110 liters/minute). The stand-offs are 0.450 inch and1.700 inches respectively. The pressure drop across the nozzle is 1138psi and the maximum impact pressure (on the close side) is 760 psi andthe minimum impact pressure (on the far side) is 136 psi.

EXAMPLE 3

Using a centralized 2.125 inch Roto-Jet™ with R90C nozzles in 4 inchinner diameter casing with a stand-off of approximately 1.22 inch, and aflow rate of 110 liters/minute through the tool in a fluid filled holegenerated a 1021 psi pressure drop across the nozzles and 254 psi impactpressure.

EXAMPLE 4

A 2.125 inch Roto-Jet™ with R90C nozzles was used to clean a typicalgravel pack with 40/60 sand with a 4 inch base pipe screen and 0.008inch gap wire wound screen. The casings size is 6.5 inch inner diameter.ROP would be ½ meters/minute. Pump rate 105 liters/minute. Impactpressure is 224 psi. This is a solvent application rate of 210liter/meter. One pass of solvent was made and then the solvent wasflushed out at the same conditions except POOH at ¼ meter/minute.

While the compositions and methods of this invention have been describedin terms of preferred embodiments, it will be apparent to those of skillin the art that variations may be applied to the process describedherein without departing from the concept, spirit and scope of theinvention. For instance, in instances where the gravel pack is pluggedwith essentially soluble plugging materials, the pack may be washed bysimply delivering a pressure pulsating treatment fluid into the gravelpack and dissolving the soluble plugging materials with the treatmentfluid. Conversely, in instances where the gravel pack is plugged withessentially insoluble plugging materials, the pack may be washed bydelivering a pressure pulsating fluid into the gravel pack and movingthe insoluble plugging materials through the gravel pack with the fluid.The insoluble materials could subsequently be circulated out of thewellbore. In this application, the fluid does not have to be a treatmentfluid since the insoluble materials are being removed by the hydraulicoscillating of the plugging materials by the pulsating fluid. It willalso be appreciated that the invention may be used to remove soluble andinsoluble fines from open hole completions and cased hole completions inwells without screens or gravel packs. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the spirit, scope and concept of the invention as it is set outin the following claims.

1. A method of cleaning a plugged or partially plugged gravel pack in awellbore comprising the steps of: using a tangential vortex to deliver atreatment fluid into the gravel pack; dissolving soluble fines in thegravel pack with the treatment fluid; moving insoluble fines through theinterstitial pore space of the gravel pack; and circulating theinsoluble fines out of the wellbore.
 2. The method of claim 1, whereinthe tangential vortex is created by delivering a pressure pulsating jetof the treatment fluid onto a gravel pack screen with one or morejetting nozzles.
 3. The method of claim 2, wherein the tangential vortexis created in the region below the one or more nozzles, therebydirecting treatment fluid behind the screen and into the gravel pack. 4.The method of claim 2, further comprising orienting the one or morejetting nozzles to provide an axial downward component to the jetdirection.
 5. The method of claim 1, wherein the treatment fluid is anacid.
 6. The method of claim 5, wherein the acid is selected fromhydrochloric or hydrofluoric acids.
 7. The method of claim 1 furthercomprising cleaning the gravel pack with about 40 liters to about 400liters of acid per meter of gravel pack.
 8. The method of claim 2wherein the pressure pulsating jet is delivered at an impact pressure ofabout 50 to about 500 psi.
 9. The method of claim 2, wherein thepressure pulsating jet is delivered at an impact pressure of about 5 psito about 850 psi.
 10. The method of claim 2, wherein the one or morejetting nozzles are lowered through the gravel pack at a rate rangingfrom about 0.2 meters per minute to about 10 meters per minute.
 11. Amethod of washing a gravel pack in a wellbore comprising the steps of:delivering a treatment fluid into the gravel pack with a pressurepulsating jet and a tangential vortex; and dissolving soluble pluggingmaterials in the gravel pack with the treatment fluid.
 12. The method ofclaim 11, wherein the tangential vortex is created by delivering thepressure pulsating jet of the treatment fluid onto a gravel pack screenwith one or more jetting nozzles.
 13. The method of claim 12, whereinthe tangential vortex is created in the region below the one or morenozzles, thereby directing treatment fluid behind the screen and intothe gravel pack.
 14. The method of claim 12, further comprisingorienting the one or more jetting nozzles to provide an axial downwardcomponent to the jet direction.
 15. The method of claim 11, wherein thetreatment fluid is an acid.
 16. The method of claim 15, wherein the acidis selected from hydrochloric or hydrofluoric acids.
 17. The method ofclaim 11 further comprising cleaning the gravel pack with about 40liters to about 400 liters of acid per meter of gravel pack.
 18. Themethod of claim 12 wherein the pressure pulsating jet is delivered at animpact pressure of about 50 to about 500 psi.
 19. The method of claim12, wherein the pressure pulsating jet is delivered at an impactpressure of about 5 psi to about 850 psi.
 20. The method of claim 12,wherein the one or more jetting nozzles are lowered through the gravelpack at a rate ranging from about 0.2 meters per minute to about 10meters per minute.
 21. The method of claim 11, further comprising movinginsoluble fines through the interstitial pore space of the gravel packand circulating the insoluble fines out of the wellbore.
 22. A method ofwashing a gravel pack in a wellbore comprising the steps of: deliveringa pressure pulsating treatment fluid into the gravel pack with apressure pulsating jet and a tangential vortex; moving insolubleplugging materials through the gravel pack with the fluid; andcirculating the insoluble plugging materials out of the wellbore. 23.The method of claim 22, wherein the soluble plugging materials aredissolved in the gravel pack with the treatment fluid.
 24. The method ofclaim 22, wherein the tangential vortex is created by delivering thepressure pulsating jet of the treatment fluid onto a gravel pack screenwith one or more jetting nozzles.
 25. The method of claim 24, whereinthe tangential vortex is created in the region below the one or morejetting nozzles, thereby directing treatment fluid behind the screen andinto the gravel pack.
 26. The method of claim 24, further comprisingorienting the one or more jetting nozzles to provide an axial downwardcomponent to the jet direction.